Field
Non-volatile memory.
Description of Related Art
Magnetoresistive RAM (MRAM) is a type of non-volatile memory that stores data in the form of magnetic storage element. The magnetic storage elements are formed typically of a magnetic tunnel junction (MTJ) which is a component of two ferromagnets separated by a thin insulating layer. One of the two ferromagnets is a permanent magnet set to a particular polarity. The magnetic field of the other ferromagnet can be changed to match that of an external field to store memory. Representatively, a cell containing such magnetic storage element may be selected by powering an associated transistor that switch its current from a supply line through the cell to ground. Due to a magnetic tunnel effect, an electrical resistance of the cell changes due to the orientation of the field in the two ferromagnets. By measuring a resulting current, a resistance inside of a particular cell can be determined (the cell can be read). Data is written to the cells representatively by inducing a magnetic field at a junction which is seen in the writable plate.
Spin transfer torque (STT) MRAM uses spin-aligned electrons to influence electrons flowing into a layer to change their spin. Where a current is passed through a magnetization layer (the fixed magnetic layer) the current will come out spin polarized. With the passing of each electron, its spin (angular momentum) will be transferred to the magnetization in the next magnetic layer, called the free magnetic layer, and will cause a small change on its magnetization. This is, in effect, a torque-causing precession of magnetization. Due to reflection of electrons, a torque is also exerted on the magnetization of an associated fixed magnetic layer. In the end, if the current exceeds a certain critical value (given by damping caused by the magnetic material and its environment), the magnetization of the free magnetic layer will be switched by a pulse of current, typically in about one to 10 nanoseconds. Magnetization of the fixed magnetic layer may remain unchanged since an associated current is below its threshold due to geometry or due to an adjacent anti-ferromagnetic layer.
Spin-transfer torque can be used to flip the magnetic storage elements in magnetic random access memory. Spin-transfer torque MRAM, or STT-MRAM, has the advantages of lower power consumption and better scalability over conventional magnetic random access memory (MRAM) which uses magnetic fields to flip the active elements. However, significant improvements are still needed in the area of STT-MRAM device manufacture and usage.